The invention relates to a method of fabricating a phase shifting reticle, and more particularly to a method of fabricating a phase shifter.
Some types of phase shifting reticles have been proposed and disclosed in July 1990 NIKKEI MICRODEVICES No. 61, pp.103-114. One of those has a following structure. A conductive layer 2 covers an entire surface of a glass plate 1 to prevent electron charging by electron-beam lithography. Opaque chrome layers 3 and apertures cover the conductive layer 2 to define a desired pattern. The apertures are alternatively covered with transparent phase shifting layers 4, or phase shifters. The lights transmitted through adjacent apertures have a different phase of 180 degree with one another.
The conventional fabrication processes of the phase shifting reticle are illustrated in FIGS. 1A to 1C. The glass plate 1 is covered with the conductive layer 2, if required. The opaque chrome layers 3 are formed on the conductive layer 2 to define the intensity pattern so that a formation of the ordinary reticle is completed. This structure is illustrated in FIG. 1A. A transparent silicon dioxide layer 4 (SiO.sub.2 layer) which serves as a phase shifter is formed on the entire surface of the reticle by use of either a chemical vapor deposition method or a spin coating method. A resist 5 is applied on the silicon dioxide layer 4 by use of the spin coating method. As shown in FIG. 1B, the resist 5 is subjected to a desired patterning by use of electron-beam lithography exposure 7, followed by a deposition. The silicon dioxide layer 4 is patterned by etching, after which the resist 5 is completely removed whereby a formation of a reticle with phase shifters is competed as shown in FIG. 1C.
When an optical wavelength of 435.8 nm is used, the phase shifter of silicon dioxide requires an even thickness of approximately 470 nm .+-.10 nm. It is required that the thickness of the phase shifter is even in the range of .+-.2 percent of a desired thickness. A precise control of the phase shifter thickness is also required.
The set forth conventional fabrication methods of the phase shifting reticle is engaged with following problems. The reticle formed by the chemical vapor deposition method has an uneven surface in the range of .+-.5 percent so that it is difficult to realize a desired optical phase shift. While, the spin coating method requires a high temperature treatment. It is required that the silicon dioxide layer 4 is sintered at 400 degree temperature. This high temperature sintering treatment causes a deformation and a peel of the opaque chrome layer. Also, the reticle formed by the spin coating method has a relatively large different thickness at a center portion and an edge portion with one another. The large different of the thickness causes a difficulty of realizing a desired optical phase shift.